1. Field of the Invention
The present invention relates in general to suspension devices, and more particularly to suspension devices of electromagnetically controlled type.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional suspension device of the above-mentioned type (which will be referred to as "electromagnetic suspension device" hereinafter) will be briefly described, which is disclosed in Japanese Patent First Provisional Publication 2-37016 as a suspension unit for a motor vehicle.
The electromagnetic suspension device shown in the publication comprises generally an outer tube which is connected to a vehicle body and a rod which is connected to a road wheel. The rod has at its upper end a piston which is axially movably disposed in the outer tube. A coiled spring is disposed between the vehicle body and the road wheel, concentrically surrounding the rod. A permanent magnet is mounted to the upper portion of the rod and a coil is disposed on an inner surface of the outer tube in a manner to spacedly surround the permanent magnet. By applying current to the coil, there is produced a driving force in the direction of the axis of the coil. Thus, by controlling the flowing direction and intensity of the current, it becomes possible to maintain the height of the vehicle.
However, due to its inherent construction, the electromagnetic suspension device of this type has a limit in absorbing shocks applied thereto. In fact, the control of the shock absorbing is effected by only the driving force produced by the coil. Thus, if an abnormally big impact force is supplied to the suspension device during running of the vehicle, it tends to occur that the device is broken. Furthermore, since the needed driving force is produced by the current applied to the coil, electric power consumption is excessive thereby deteriorating the fuel consumption of the vehicle.